1. Field of the Invention
The present invention relates to an image display device and an image display method, and particularly to an image display device, which includes a light-emitting element of a current drive type in each of pixels, and executes gray-scale expression based on a digital signal.
2. Description of the Background Art
As an image display device of a flat-panel type, attention has been given to an image display device of a self-light-emitting type, in which each pixel is formed of a light-emitting element of a current drive type. The image display device of the self-light-emitting type has high visibility as well as high moving picture quality. A light-emitting diode (LED) is well known as a kind of light-emitting element of the current drive type.
The image display device includes a plurality of pixels, which are arranged in rows and columns, and are successively driven by dot-sequential scanning or line-sequential scanning to receive a display current. Each pixel keeps luminance corresponding to the display current, which was received for the above driving, until its next driving. The display current received by each pixel is usually formed of an analog current for achieving gray-scale expression. This analog current can be set to a level intermediate between maximum and minimum luminance levels of each light-emitting element so that each pixel can execute the gray-scale expression.
Therefore, the image display device provided with the light-emitting elements of the current drive type requires a current supply circuit, which generates the display current according to image data for exhibiting stepwise luminance at each pixel.
For example, Japanese Patent Laying-Open No. 11-212493 has disclosed a structure of an image display device, which provides image data formed of a digital signal of multiple bits. For supplying a display current (i.e., gray-scale current) for gray-scale expression, this structure includes a plurality of constant current supplies or sources connected in parallel as well as thin film transistors (TFTs), which are turned on/off in response to a plurality of bits forming the image data, and are interposed between the constant current supplies. A sum of currents selectively supplied from these constant current supplies is supplied to the light-emitting element.
In particular, a ratio of powers of 2 (e.g., 1:2:4:8) is set between output currents of the constant current supplies so that the gray-scale current can be controlled stepwise at equal intervals.
However, the current supply circuit described above suffers from a problem that a current supply circuit increases in size with the number of bits of the image data. Typically, the number of constant current supplies increases so that a footprint or area of the current supply circuits increases, and outer sizes of the image display device increase. Further, increase in circuit scale results in an increased manufacturing cost.